1. Field of the Invention
The present invention relates to a pattern inspection method and a pattern inspection apparatus, and in particular relates to a pattern inspection method and a pattern inspection apparatus which are used for detecting a defect formed on a phase shift mask or the like used by a lithography technology.
2. Description of the Related Art
In recent years, with increase of integration degree of a semiconductor integrated circuit, a lithography technology capable of forming a finer pattern than a conventional one has been needed. As one of lithography technologies capable of forming a fine pattern, a lithography technology using Phase Shift Mask (PSM) is known. There are two kinds of PSM, an Attenuated PSM (hereinafter, Att-PSM) in which a member for shifting a phase of an exposure light is arranged to one of adjacent light transmission patterns, and an Alternating PSM (Alt-PSM) including a trench on a base of the photomask. In the PSM, a high contrast can be obtained by using interference between transmitted lights which have different phase from each other. The Att-PSM may be described as Half Tone PSM, and the Alt-PSM may be described as Levenson PSM. Conventionally, a photomask used for a usual lithography is a binary mask. The binary mask is the photomask on which a light shielding pattern made of a metallic film or the like on a glass substrate is formed. The binary mask does not shift a phase of an exposure light.
In order to expose a fine pattern by using a phase shift mask, it is important that a position and a size of a usual pattern for a light shielding and a phase shifter are exactly consistent with design data. The phase shifter is a part of a pattern of a photomask and is a pattern for shifting a phase of a transmitted light. Therefore, a method for detecting a defect is required, the defect meaning that a position and a size of a pattern of a photomask are not consistent with design data. In recent years, inspection technologies for a photomask, e.g. a phase shift mask, using an interference effect of light are desired.
Under such background, various methods for inspecting a photomask have been invented. A method for inspecting a phase shift of the PSM includes, for example, a method using interference of light. For example, a method using an optical heterodyne interferometry is disclosed in Japanese Patent Application Laid-Open No. Hei 6-331321. In Japanese Patent Application Laid-Open No. 2002-287327, a method using a differential interference microscope is disclosed. A defect of a phase shifter is described as “phase defect”, hereinafter.
As one of methods for inspecting a phase defect of a PSM that use diffraction and scattering of light, there is a method which uses an analysis of a Fourier transform image of a transmitted light having passed through a phase shift pattern or a reflected light having been reflected by the phase shift pattern, wherein the PSM is uniformly illuminated. This method is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 4-229863. Additionally, there is a detection method in which only a scattered and diffracted light having been scattered and diffracted by a phase shift pattern are detected through a spatial filter on a Fourier transform plane, wherein the a phase shift pattern is obliquely illuminated. This method is described in Published Japanese translation of PCT application No. 2002-519667.
As one of methods to detect a mask pattern defect with a two-dimensional shape, there is a Scanning Optical Microscopy (SOM). In an SOM, a light beam is concentrated on a surface in which a pattern of a photomask is formed and the pattern is scanned. A pattern shape is observed by detecting a transmitted light and a reflected light from the surface. Since an SOM has a feature of concentrating light, a high S/N ratio can be obtained. Therefore, an SOM is particularly advantageous for a reflective inspection. A pattern defect with a two-dimensional shape is described as “shape defect” hereinafter.
In Japanese Patent Application Laid-Open No. Hei 7-083620, a laser displacement meter for measuring a very small height of an object is described. Usually, electronic components whose heights are different each other are mounted on a printed wiring board. Additionally, a reflectivity of a surface of a printed wiring board is not uniform. This laser displacement meter can measure a small height difference on a surface of a printed wiring board. This laser displacement meter includes a light projection optical system which scans an object with a laser beam and a light receiving optical system which receives a reflected light from the object. The light receiving optical system concentrates a reflected light including astigmatism and receives the reflected light with a quadrant detector to measure a height of the object. In Japanese Patent Application Laid-Open No. Hei 9-257444, a disk test apparatus which applies a laser beam to a disk surface, receives a reflected light from a disk by an astigmatic method and detects a distance from a reflecting surface by using a quadrant detector is disclosed.
However, the above-mentioned conventional art has a problem as shown below.
In the technology described in Japanese Patent Application Laid-Open No. Hei 6-331321 and Japanese Patent Application Laid-Open No. 2002-287327, two light beams whose positions are slightly displaced from each other are applied to an object and interference of two lights is utilized. Therefore, detection ability depends on a direction where two beams are displaced. Additionally, a detectable range of a pattern line width depends on a distance between two displaced beams.
A technology described in Japanese Patent Application Laid-Open No. Hei 4-229863 is a method in which a phase variation amount, that is only a film thickness of an object, is basically measured and a very small defect of a pattern is not an object to be detected. This technology has been invented based on an analysis of a Fourier transform image in a general imaging method using a uniform illumination like a projection exposure apparatus. Therefore, a control of an illumination has to be correctly performed so as to uniformly illuminate both of regions where a phase shifter is added and where a phase shifter is not added.
The technology described in Published Japanese translation of PCT application No. 2002-519667 is one of methods which are generally used for detecting a defect of a semiconductor wafer etc. That is, in this technology, a defect is detected by measuring and analyzing a scattered light or a diffracted light from an area which is illuminated by dark field illumination or bright field illumination by using a light source of a laser or the like. Various methods have been invented as a technology to serve the same purpose as mentioned above. In these methods, S/N ratio of a very weak scattered light and diffracted light from a microscopic defect has to be improved. Published Japanese translation of PCT application No. 2002-519667 discloses a detection method in which in order to improve S/N ratio and determine a type of a defect, a spatial filtering is performed to a diffracted light in a far field region. In this method, various scattered lights or diffracted lights are generated based on a pattern shape and a defect shape of an object to be detected. Therefore, in order to capture the scattered light or diffracted light, setting of an angle of an illumination light, arrangement of a light receiving system, selection of a spatial filter type or the like, which are complicated, are required.
A defect detection apparatus using an SOM detects only a total amount of light. Therefore, the apparatus lacks detection ability for a pattern defect having a reflectivity which is same as the other part at a wavelength of an inspection light. The apparatus lacks detection ability to detect a defect which changes only a phase of light such as a phase defect.
Specifically, a phase defect of the Alt-PSM means a defect in which a trench part in a photomask base is formed at an incorrect position or means a defect in which the trench part is not formed at a correct position. Thus, a phase defect of the Alt-PSM is located on a glass substrate of a photomask. A region having a phase defect and a normal region are formed with glass material. Therefore, a reflectivity at a wavelength of an inspection light at a defect region is equal to a reflectivity at a normal region.
A phase defect of the Att-PSM is a defect in which a translucent light shielding film for shifting a phase of an exposure light is formed at a region where the film is not needed or a defect in which the film is not formed at a region where it is needed. Also, a phase defect includes a defect in which a thickness of a light shielding film is not a normal even though the light shielding film is formed at an appropriate region. In this case, a reflectivity at a wavelength of an inspection light at a defective region is equal to a reflectivity at a normal region.
In the technology described in Japanese Patent Application Laid-Open No. Hei 7-083620 and Japanese Patent Application Laid-Open No. Hei 9-257444, a height of an object or a flatness of a disk can be detected by using an astigmatic method. An astigmatic method is a well-known method that is commonly used for an automatic focus adjustment or the like. In the technology, a phase defect of a photomask can be detected using an astigmatic method. However, both of a shielding pattern on a photomask and a phase distribution of a reflected light cannot be detected.